[Zubersoft]

There are only minor differences between MobileSheets on each platform, but the significance of those differences depends on how you want to use the app. The main thing is that, on Android, you have the option of managing all your files and folders yourself. On iOS, this is not possible at all (the operating system only grants MobileSheets access to the folder it was assigned). On Windows, you can at least change the storage location, but the file permissions still requires that every file that the app uses is placed under the storage location, so it's still more restrictive than Android. In general, I would recommend Android or iOS. From a hardware perspective, the iPad Pro is hard to beat given the size and aspect ratio, but it is incredibly expensive. With Android e-ink tablets, the BOOX tablets are also 4:3 and are even larger at 13.3", but they are also expensive (and not every user wants e-ink). The Samsung S7+, S8+ and S9+ are all tremendous Android tablets, as is the S8 Ultra/S9 Ultra if you want the largest size possible, but it's also very much a widescreen format. All of the tablets I mentioned come with styluses/pens/pencils that work incredibly well for annotating. 

The companion app can connect to any of the platforms (Android, Windows and iOS) without any issues. So you don't need to have any concerns about that. The companion app is getting a major face lift with the next update as well, as I've ported it over to a cross-platform framework and I'm updating the UI as we speak to use a material design theme.

If I had to pick, I think the Android version is overall the most stable and best-performing with iOS close behind it. The Windows version works perfectly fine, but is held back a little by Microsoft's UWP framework (I'm looking to switch away from this at some point). 

I'm sure others will have their own opinions to share, as my perspective is a little different as the developer of the app.

Mike